I use some shortcuts frequently, mainly ALT+F4 to close windows and ALT+F2 to call QuickSand (a KDE text-entry-based launcher).

Some days ago, I found that pressing ALT+Fn leaves the KDE VT and switches to one of the text-based Virtual Terminals.
I.e.: pressing ALT+F2 gets me to VT2, then pressing either ALT+F7 or CTRL+ALT+F7 brings up KDE again (where the QuickSand launcher has already started - so it still reacts correctly to the keystrokes, but they are "caught" earlier by some other process).

Intended behaviour is to switch to a VT only on CTRL+ALT+Fn.

Can anybody give a hint which component/package is responsible for catching the VT-switching keystrokes (if needed, I can provide apt-get log files)?
What should I do to restore the original behaviour (where is it configured)?